Add group by support for tags.

## Overview

This commit adds GROUP BY support for tags. Previously only the interval could be grouped by.


## Queries

This query is now supported:

```
SELECT sum(value)
FROM cpu
WHERE time >= now() - 3h
GROUP BY time(1h), host
```
Is this global change from int64 -> string because string can store any GROUP BY? Before it was just interval, which is always numeric? Is that correct?